Well let’s give some counter examples in the softwares I mentioned :

WhatsApp closed : Owned by Facebook. Well Facebook had multiple data leaks, privacy violations and nothing substantial was done about it. Definitely not trustable (also zero days are getting sold on the black market for WhatsApp (techcrunch.com/…/zero-days-for-hacking-whatsapp-a… ).

Telegram closed : not end to end encrypted. Russian app. Not trustable.

Signal open (servers I don’t know): well this one is e to e encrypted. Open source, maybe could be trusted. Seems to have passed some security audits (community.signalusers.org/t/…/13243), tho it’s based in the US and uses servers, maybe the US may have super computers capable of decrypting such communications. However is signal has switched their encryption to quantum computer resistance it may be too hard even for a state actor.

Olvid (open, servers I don’t know) : is French and why not use a local messaging app witch also is very secure and open source.

Notice how closed source is untrusted here. The economic activity of the tool changes how trustable it is. Military équipement has a huge and strict budget, it has to be secure.

Communication apps are user first. So they do what they can get away with, and that is very true for Facebook.